Sayyiduna Anas (may Allah be pleased with him) relates: When the Prophet (peace and blessings be upon him) prostrated, none of us would prostrate until he saw that he had gone into prostration.
Al-Bazzar recorded it, and Abu Ya'la recorded something similar. In al-Bazzar's narration there is Sa'id bin al-Mufaddal, whom Abu Hatim declared weak and others declared thiqah (trustworthy). Abu Ya'la's narration is munqati' (disconnected) between al-A'mash and Sayyiduna Anas (may Allah be pleased with him).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2409