Ahmad ibn al-Khalil said that he heard Ishaq ibn Ibrahim {Ibn Rahawayh} say:
“When {Imams} ath-Thawri رحمه الله, Malik رحمه الله and al-Awza’i رحمه الله agree on a matter, it is Sunnah, even if there is no text on it.”
{Tadhkirah al-Huffaz}
~ May Allah have mercy on them.