Sunan ad-Darimi

Mus'ab bin Sa'd narrated that when the Banu Abdullah bin Mas'ud would bow (in ruku'), they would place their hands between their thighs. So I prayed next to (my father) Sa'd (bin Abi Waqqas) and did the same, but he struck my hand. When the prayer was finished, he said: "Son, place your hands on your knees (in ruku')." The next day, I again prayed next to him and did the same, and again he struck my hand. When the prayer was finished, he said: "We used to place (our hands joined) between our thighs like this, but we were commanded to place our palms on our knees."

Muhammad bin Yusuf reported that Isra'il narrated from Abu Ishaq, who narrated from Mus'ab in the same manner.

Sayyiduna Abu Mas’ud Ansari (may Allah be pleased with him) said: Shall I not lead you in prayer as the Messenger of Allah (peace and blessings be upon him) used to pray? (Salim al-Barad said:) So he said the takbir, performed ruku‘, and placed both his hands (palms) on both his knees, and kept his fingers spread apart until every joint became straight.