It is narrated from Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) recited Sad [Surah Sad: 1] on the pulpit. When he reached the verse of prostration, he came down from the pulpit and prostrated, and the people prostrated with him (peace and blessings be upon him). Then on another day he (peace and blessings be upon him) recited Sad [Surah Sad: 1], and when he reached the verse of prostration, the people got ready to prostrate. The Messenger of Allah (peace and blessings be upon him) said: "This is only the repentance of a prophet, but I have seen that you have got ready to prostrate." Then he (peace and blessings be upon him) came down (from the pulpit) and prostrated, and the people also prostrated with him (peace and blessings be upon him).
This hadith has a hasan (good) chain and is sahih (authentic).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3740