The source that when an abortion needs to be performed it should be done by a female Jewish physician is the Tzitz Eliezer, Rav Eliezer Waldenberg zt”l. Rav Waldenberg was considered to be the foremost posek in Eretz Yisrael on medical-halachic issues. I cannot be sure, but it seems reasonable that Rav Leff bases his opinion on the Tzitz Eliezer.
The Tzitz Eliezer’s opinions on abortion can be found in Chelek 8, 9, 13,14 and 15. In Chelek 8 he has a long piece on the subject,in the others he discusses specific situations.