Short answer
It is better not to. RIP comes from a Christian prayer formula, and Muslims have their own words: Inna lillahi wa inna ilayhi raji'un, and a dua such as Allah yarhamhu (may Allah have mercy on him). If someone says RIP meaning a sincere prayer, it is not a sin, but use the sunnah words yourself.
The sunnah when hearing of a death is istirja' (Inna lillahi wa inna ilayhi raji'un) and dua for the deceased's forgiveness and mercy. Phrases taken from another religion's rites are best avoided; saying RIP with the intention of a dua is not sinful in itself. For a non-Muslim, asking for forgiveness is not allowed.
The four schools agree on this.
Explanation
On social media, "RIP" appears under every death announcement, and many Muslims type it without thinking. The phrase comes from the Latin requiescat in pace, part of Christian funeral prayers. Its meaning, a wish for peace for the dead, is not wrong in itself. But Islam has given us far richer words, taught by the Prophet ﷺ, and using them is both sunnah and a dua that reaches the person.
What to say instead:
- Inna lillahi wa inna ilayhi raji'un: "We belong to Allah and to Him we return." See its meaning.
- Allah yarhamhu (for a man) or Allah yarhamha (for a woman): "May Allah have mercy on him/her."
- Allahumma-ghfir lahu warhamhu: "O Allah, forgive him and have mercy on him."
- In English: "May Allah forgive her and grant her Jannah."
When Abu Salamah died, the Prophet ﷺ made dua for him by name: "O Allah, forgive Abu Salamah, raise his rank among the guided..." That is the model: a real prayer, by name, for forgiveness and mercy.
If others say RIP. Do not scold grieving people. Most mean a sincere prayer, and scholars do not count it a sin when meant that way. Simply write the Islamic words yourself.
For a non-Muslim. A Muslim does not ask Allah to forgive someone who died outside Islam, so RIP, which is a prayer for their hereafter, is avoided. You can say "I am sorry for your loss" and "May God give you patience" to the family; see dua for a non-Muslim who died and what to say when someone dies.
The evidence
Quran, Surah Al-Baqara 2:156
ٱلَّذِينَ إِذَآ أَصَٰبَتْهُم مُّصِيبَةٌۭ قَالُوٓا۟ إِنَّا لِلَّهِ وَإِنَّآ إِلَيْهِ رَٰجِعُونَ ﴿١٥٦﴾
Who, when disaster strikes them, say, "Indeed we belong to Allah, and indeed to Him we will return."
The words a believer says at a death.
Quran, Surah Al-Hashr 59:10
وَٱلَّذِينَ جَآءُو مِنۢ بَعْدِهِمْ يَقُولُونَ رَبَّنَا ٱغْفِرْ لَنَا وَلِإِخْوَٰنِنَا ٱلَّذِينَ سَبَقُونَا بِٱلْإِيمَٰنِ وَلَا تَجْعَلْ فِى قُلُوبِنَا غِلًّۭا لِّلَّذِينَ ءَامَنُوا۟ رَبَّنَآ إِنَّكَ رَءُوفٌۭ رَّحِيمٌ ﴿١٠﴾
And [there is a share for] those who came after them, saying, "Our Lord, forgive us and our brothers who preceded us in faith and put not in our hearts [any] resentment toward those who have believed. Our Lord, indeed You are Kind and Merciful."
Believers pray for forgiveness for those who died before them.
Sahih Muslim 920
Sahih... اللهم اغفر لأبي سلمة وارفع درجته في المهديين ...
"O Allah, forgive Abu Salamah and raise his rank among the rightly guided."
The sunnah is a real dua for the dead person by name.
Where the ruling is recorded
- Radd al-Muhtar (Ibn Abidin), Kitab al-Salah, Bab Salat al-Janazah, condolence. Istirja' and dua for the dead at the news of a death.
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.