It is reported from Al-Sha’bī that he said:

Zayd b. Thābit – Allāh be pleased with him – once mounted [his horse or camel] to ride it, so Ibn ‘Abbās – Allāh be pleased with them – took hold of the reins [to serve him by leading his ride]. When he did this, Zayd said (out of respect for him), “Don’t do this o cousin of Allāh’s Messenger – Allāh’s praise and peace be upon him.” Ibn ‘Abbās said, “This is what we have been told to do with our scholars (i.e. respect them and serve them).” Zayd said, “Show me your hand.” Ibn ‘Abbās held out his hand, and Zayd kissed it and said, “And this is what we have been told to do with the Ahl Al-Bayt (Family) of our Prophet – Allāh’s praise and peace be upon him.”

Abū Bakr Al-Daynūrī, Al-Mujālasah wa Jawaāhir Al-‘Ilm 4:146, et al