Nas has filed legal docs asking a judge to penalize Kelis because she refused to turn over 8-year-old Knight ... she says it's because the kid wanted to stay with her during Passover. Nas says it was his weekend and the whole Passover thing doesn't make sense.

He says she's been difficult generally when it comes to getting Knight. He says once when he arrived at her house, she screamed at him telling him he couldn't take the child. Nas says Kelis has refused to let him pick up the child from school on several occasions.

He's asking for $22,000 for all the lawyers' bills he's had to pay when she's allegedly violated their custody agreement.