Short answer
If the imam makes a mistake in an action, such as standing up instead of sitting, men say "SubhanAllah" and women clap the back of one hand on the other palm. If he gets stuck or makes a serious error in recitation, a follower behind him may prompt him with the right words. In the Hanafi school this does not break the prayer, but prompting anyone other than your own imam does.
Prompting your own imam (fath or luqmah) is permissible and does not break the follower's or the imam's prayer. A follower should not rush to prompt for small slips, and the imam should move to another verse or surah rather than stand silently forcing a prompt. Prompting someone who is not your imam, or an imam taking a prompt from someone not in his prayer, breaks the prayer.
Other madhhabs
- Maliki
- Women also say "SubhanAllah" to alert the imam; clapping is disliked for them.
Explanation
The Prophet ﷺ once made a mistake in prayer and said afterwards that he was human, forgetting as others forget, so when he forgot they should remind him. He also taught that saying "SubhanAllah" is for men and clapping is for women. These are the two tools of the followers.
Mistakes in actions:
- Men: say "SubhanAllah" clearly enough for the imam to hear.
- Women: clap lightly, striking the back of the right hand on the palm of the left.
- Standing for an extra rakah: alert him. If he returns, follow him. If he does not, do not follow him into the extra rakah; wait seated and ask the imam afterwards, since the ruling depends on whether he had already sat for the final tashahhud. See forgetting a rakah or praying an extra one.
- Standing up from the first tashahhud: once he is fully or nearly upright, he should not return, and he does sajdah sahw at the end. Follow him.
Mistakes in recitation:
- The imam gets stuck: a follower who knows the passage may recite the next words for him.
- A mistake that changes the meaning: prompt him with the correct words.
- A minor slip: such as skipping a verse or a small tajweed mistake, let it pass; see mistakes in recitation.
- For the imam: if you lose your place, it is better to move on to another verse or go into ruku than to wait for a prompt.
Who may prompt: only someone praying behind that imam. If someone praying separately, or not praying at all, prompts the imam and he takes it, his prayer is affected in the Hanafi school. See the follower and sajdah sahw and sajdah sahw.
The evidence
Sahih al-Bukhari 1203
Sahih... التسبيح للرجال والتصفيق للنساء ...
"Saying SubhanAllah is for men, and clapping is for women."
How followers alert the imam to a mistake.
Sahih Muslim 572
Sahih... إنما أنا بشر أنسى كما تنسون فإذا نسيت فذكروني ...
"I am only human; I forget as you forget, so when I forget, remind me."
Followers may remind the imam.
Where the ruling is recorded
- al-Hidaya, Kitab al-Salah, chapter on what invalidates the prayer. Prompting one's own imam does not invalidate; prompting someone else does.
- Radd al-Muhtar (Ibn Abidin), Kitab al-Salah, chapter on what invalidates the prayer. The follower should not hurry to prompt, and the imam should not force it by pausing.
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.