“And take in thy hand a little grass, and strike therewith: and break not (thy oath).” Truly We found him full of patience and constancy. How excellent in Our service! ever did he turn (to Us)!
In the Hebrew Bible we learn that Job was patient when afflicted with suffering but that his wife was bitter and even taunted him. (See Job 2:9-10). Islam has a different take on her behavior, finding her more noble than that. Commentators of this verse, using traditions of the Prophet to guide them, have understood that during the darkest hour of his affliction, Job, in his frustration with his wife (when she was apparently tricked by Satan into saying something spiteful to him), swore that he would hit her 100 times with a stick if he ever recovered from the loss of his children, health and fortune. (Ahmad) During his many years of suffering, however, his wife remained by his side and cared for him, so how could he keep such a foolish oath? When God blessed him with more than he had before (as a reward for his patience,) Job felt remorseful for his hasty promise. God gave Job a way to fulfill his oath without actually physically hurting his wife, as no prophet of God would ever hurt a woman so. Thus, he would hit her with “a few harmless blades of grass.” (Ibn Kathir)
Emerick, Yahiya. The Meaning of the Holy Qur’an in Today’s English (p. 829). Unknown. Kindle Edition.