Al-Ansari (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) instructed Ja'far (may Allah be pleased with him) in the same way, and said: "In the second prostration of the first rak'ah as well," as in the hadith of Mahdi bin Maymun.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4920