Musnad Ahmad

Once Alqama and Aswad both came to the service of Sayyiduna Ibn Mas'ud (may Allah be pleased with him). When the time for prayer came, Alqama and Aswad stood behind him. Sayyiduna Ibn Mas'ud (may Allah be pleased with him) took their hands and made one stand on his right side and the other on his left side. Then, when both of them bowed (in ruku'), they placed their hands on their knees. Sayyiduna Ibn Mas'ud (may Allah be pleased with him), seeing this, struck their hands and joined his own hands together, interlacing his fingers, and placed both hands between his thighs, and said: "I saw the Prophet (peace and blessings be upon him) doing it this way." Benefit: This act is called «تطبيق», and in the beginning, this was the method of ruku'. Later, this ruling was abrogated, but Sayyiduna Ibn Mas'ud (may Allah be pleased with him) did not accept its abrogation until the end.