Short answer
Not necessarily. The Quran says the believers' mark is on their faces from the trace of prostration, but the early commentators explained this mainly as the light, humility and good character that prayer leaves, not a callus. A mark that comes naturally from long prayer is fine. Making one on purpose to look pious is showing off.
There is no ruling requiring or recommending a mark on the forehead. A mark that appears without effort is not blameworthy, deliberately rubbing the forehead to create one is riya' (showing off) and sinful, and having no mark says nothing about a person's prayer.
The four schools agree on this.
Explanation
Allah describes the Companions in Surah al-Fath as people you see bowing and prostrating, seeking His bounty and pleasure, with "their mark on their faces from the trace of prostration". People often take this to mean the dark patch some worshippers have on their foreheads. The early scholars of tafsir gave a wider meaning.
- Ibn Abbas is reported to have explained it as good conduct and character.
- Mujahid and others said it is humility and a gentle manner, not a mark on the skin.
- Others said it is the light that will shine on the faces of those who prayed, on the Day of Judgement.
The Prophet ﷺ said that Allah has forbidden the Fire from consuming the traces of prostration on a believer, which shows the honour of the places of sajdah, whatever they look like.
What this means in practice:
- A mark from many years of long prayer, especially on rough mats, is natural and nothing to be ashamed of.
- Pressing or rubbing the forehead to create a mark is a form of showing off, which spoils deeds. Several early scholars disliked it for that reason.
- Do not judge others by their foreheads, either way. Many people of great piety have no mark at all.
- The real sign of prayer is in how it changes you: the Quran says prayer keeps a person away from indecency and wrongdoing.
For the correct way to make sajdah, see which body parts must touch the ground. For the wider meaning of prayer, see why Muslims pray five times a day.
The evidence
Quran, Surah Al-Fath 48:29
مُّحَمَّدٌۭ رَّسُولُ ٱللَّهِ ۚ وَٱلَّذِينَ مَعَهُۥٓ أَشِدَّآءُ عَلَى ٱلْكُفَّارِ رُحَمَآءُ بَيْنَهُمْ ۖ تَرَىٰهُمْ رُكَّعًۭا سُجَّدًۭا يَبْتَغُونَ فَضْلًۭا مِّنَ ٱللَّهِ وَرِضْوَٰنًۭا ۖ سِيمَاهُمْ فِى وُجُوهِهِم مِّنْ أَثَرِ ٱلسُّجُودِ ۚ ذَٰلِكَ مَثَلُهُمْ فِى ٱلتَّوْرَىٰةِ ۚ وَمَثَلُهُمْ فِى ٱلْإِنجِيلِ كَزَرْعٍ أَخْرَجَ شَطْـَٔهُۥ فَـَٔازَرَهُۥ فَٱسْتَغْلَظَ فَٱسْتَوَىٰ عَلَىٰ سُوقِهِۦ يُعْجِبُ ٱلزُّرَّاعَ لِيَغِيظَ بِهِمُ ٱلْكُفَّارَ ۗ وَعَدَ ٱللَّهُ ٱلَّذِينَ ءَامَنُوا۟ وَعَمِلُوا۟ ٱلصَّٰلِحَٰتِ مِنْهُم مَّغْفِرَةًۭ وَأَجْرًا عَظِيمًۢا ﴿٢٩﴾
Muhammad is the Messenger of Allah; and those with him are forceful against the disbelievers, merciful among themselves. You see them bowing and prostrating [in prayer], seeking bounty from Allah and [His] pleasure. Their mark is on their faces from the trace of prostration. That is their description in the Torah. And their description in the Gospel is as a plant which produces its offshoots and strengthens them so they grow firm and stand upon their stalks, delighting the sowers - so that Allah may enrage by them the disbelievers. Allah has promised those who believe and do righteous deeds among them forgiveness and a great reward.
The verse behind the question, describing the believers.
Quran, Surah Al-Ankaboot 29:45
ٱتْلُ مَآ أُوحِىَ إِلَيْكَ مِنَ ٱلْكِتَٰبِ وَأَقِمِ ٱلصَّلَوٰةَ ۖ إِنَّ ٱلصَّلَوٰةَ تَنْهَىٰ عَنِ ٱلْفَحْشَآءِ وَٱلْمُنكَرِ ۗ وَلَذِكْرُ ٱللَّهِ أَكْبَرُ ۗ وَٱللَّهُ يَعْلَمُ مَا تَصْنَعُونَ ﴿٤٥﴾
Recite, [O Muhammad], what has been revealed to you of the Book and establish prayer. Indeed, prayer prohibits immorality and wrongdoing, and the remembrance of Allah is greater. And Allah knows that which you do.
The true effect of prayer is on a person's conduct.
Sahih al-Bukhari 806
Sahih... وحرم الله على النار أن تأكل أثر السجود ...
"Allah has forbidden the Fire to consume the traces of prostration."
The places of sajdah are honoured, whether or not a visible mark exists.
Where the ruling is recorded
- Tafsir Ibn Kathir, Tafsir of Surah al-Fath, verse 29. Reports from Ibn Abbas and Mujahid that the mark is good conduct and humility.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.