Translation:

91. Fulfil the covenant of Allah when ye have entered into it and break not your oaths after ye have confirmed them; indeed ye have made Allah your surety; for Allah knoweth all that ye do.

Notes (Tafseer)

2128. The immediate reference may or may not be to the oath of fidelity to the Prophet taken at 'Aqaba fourteen months before the Hijra and repeated a little later: see v. 7, and n. 705. But the general meaning is much wider. And this may be viewed in two aspects (1) Every oath taken, or covenant made, is a Covenant before Allah, and should be faithfully observed. In this it approaches in meaning to v. 1. (2) In particular, every Muslim makes, by the profession, of his Faith, a Covenant with Allah, and he confirms that Covenant every time he repeats that profession. He should therefore faithfully observe the duties taught to him by Islam.