Musannaf Ibn Abi Shayba 3:207

Abu Bakr narrated to us, he said: Hafs narrated to us, from Mu'qil ibn Ubayd Allah, he said: I asked 'Ata' about the minimum that is permissible for bowing (ruku') and prostration (sujud). He said: "When he places his forehead upon the ground and places his hands upon his knees."