Imaam Dhahabi (rahimahullaah) stated: And it has reached us that Abu Daawud was from the diligent scholars who acted upon his knowledge, until some of the Imaams stated: Abu Daawud used to resemble Ahmad Bin Hanbal (in the manner in which he followed) the guidance, in conduct and character. Ahmad used to resemble Wakee in these (affairs) and Wakee used to resemble Sufyaan. Sufyaan used to resemble Mansoor in these (affairs) and Mansoor used to resemble Ibraaheem. Ibraaheem used to resemble Alqamah in these (affairs) and Alqamah used to resemble Ibn Mas’ood. And Alqamah said: Ibn Mas’ood used to resemble the Prophet (sallal-laahu-alayhi-wasallam) in the manner he followed the guidance and in his conduct.
[Source: Tadh-kirah Al-Huffaadh 2/592 in the Biography of Imaam Abu Daawud As-Sijistaanee]