Midrash su Salmi 141:5

“For by You I run upon a troop...” (Tehillim 18:30) R’ Chiyah said in the name of R’ Levi: when David went to wage war against the Jebusites, he began by saying “Whoever smites the Jebusites and reaches the tower…” (Shmuel II 5:8) and in another verse he says “Whoever smites the Jebusites first will be a chief (l’rosh) and an officer…” (Divre HaYamim I 11:6) So what did Yoav do? He brought a fresh cypress tree (brosh) and fixed it next to the wall of the city. He bent back the flexible head of the cypress, climbed up on to David’s head, grabbed on to the top of the tree and swung onto the top of the wall. David said “May a righteous man strike me with kindness and reprove me (yochicheni rosh)…” (Tehillim 141:5) What did the Holy One do? He lowered the wall and David went up after Yoav, as it says “…and by my God I scale a wall.” (Tehillim 18:30)