Leviticus 19:18
16 You must not go about as a slanderer among your people. You must not stand idly by when your neighbor’s life is at stake. I am the Lord. 17 You must not hate your brother in your heart. You must surely reprove your fellow citizen so that you do not incur sin on account of him. 18 You must not take vengeance or bear a grudge against any of your people, but you must love your neighbor as yourself. I am the Lord. 19 You must keep my statutes. You must not allow two different kinds of your animals to breed together, you must not sow your field with two different kinds of seed, and you must not wear a garment made of two different kinds of material.
James 5:9
7 So be patient, brothers and sisters, until the Lord’s return. Think of how the farmer waits for the precious fruit of the ground and is patient for it until it receives the early and late rains. 8 You also be patient and strengthen your hearts, for the Lord’s return is near. 9 Do not grumble against one another, brothers and sisters, so that you may not be judged. See, the judge stands before the gates! 10 As an example of suffering and patience, brothers and sisters, take the prophets who spoke in the Lord’s name.
Notes and References
"... the qualifying prepositional phrase "against one another" indicates that the concern is for attitudes and relationships within the church. "Groaning" against one another surely is a reference to complaining about one another, which is in fact a kind of judging, and so this verse is an application of both James's and Jesus's prohibitions against judging each other (James 4:11-12; Matthew 7:1). Grumbling is the opposite of patience and thus also is a mark of unbelief; it is contrary to genuine faith. Further, as Johnson suggests, we have here another connection to Leviticus 19:18 (cited by James earlier), the first part of which commands the Israelites not to bear grudges against one another (though a different word is used in the LXX of Leviticus) ..."
Davids, Peter H. James (p. 242) Baker Publishing Group, 2011