When one of you bows, let him spread his forearms over his thighs and bend forward, then place his palms together. It is as if I am looking at the alternating fingers of the Messenger of Allah ﷺ. He said, he then placed his palms together and showed them.
حدثنا أبو معاوية حدثنا الأعمش عن إبراهيم عن الأسود وعلقمة
إذا ركع أحدكم فليفرش ذراعيه فخذيه وليجنأ ثم طبق بين كفيه فكأني أنظر إلى اختلاف أصابع رسول الله ﷺ قال ثم طبق كفيه فأراهم
Its isnad is sahih. Muslim related it 1:150 at length within a longer story, through the route of Abu Muawiya from Amash. Nasai, Darimi, Hakim, and Bayhaqi also related it. See Nasb Raya 1:374, and Dhakhair Mawarith, 4860.As for "liyajna," this is how it is marked in Sahih Muslim, with a fatha on the ya, a silent jim, and ending with a hamza. Ibn Athir mentioned it under the letter ha (the unvoiced one) as "liyahna," and said, "This is how it appears in the hadith. If it is with ha, then it comes from hana meaning 'his back,' that is, when he bent it. If it is with jim, then it comes from janaa, meaning when a man leans over something and bows down upon it. The two meanings are close to each other." What we read in the book of Muslim was with jim, and in the book of Humaydi it was with ha. See Nawawi's commentary on Muslim, 5:16-17.This bending down during ruku was something Ibn Masud used to hold as his practice, but it has been abrogated in favor of holding the knees. The evidence for its abrogation is the hadith of Saad bin Abu Waqqas, which has already passed at 1570. See also 4045, 4272, 4386.