Sayyiduna Ibn Abbas (may Allah be pleased with them both) used to teach us the bowing (ruku') just as the Messenger of Allah (peace and blessings be upon him) used to teach his Companions. Then he would remain still in ruku' before us in such a way that if a drop of water were dripped between his shoulders, it would roll neither forward nor backward.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12700