Riyad as-Salihin 663
Ibn 'Umar (May Allah be pleased with them) reported:
Whenever we took a pledge of allegiance to Messenger of Allah (ﷺ) to hear and obey, he (ﷺ) would say to us, "As far as you are capable of." [Al-Bukhari and Muslim] .
وعنه قال‏:‏ كنا إذا بايعنا رسول الله صلى الله عليه وسلم على السمع والطاعة يقول لنا‏:‏ “فيما استطعتم” ‏(‏‏(‏متفق عليه‏)‏‏)‏