Midrash for Makkot 6:17

MACCOTH (Fol. 3b) Samuel said to R. Mathna: "You shall not sit down until you have explained to me the sources upon which is based the Halacha of the Rabbis which says that if one lends money to his neighbor he shall not demand the loan before the expiration of thirty days, no matter whether it was a verbal or written loan." Whereupon he answered: "From the following passage (Deut. 15, 9) The seventh year, the year of release, etc. Is it not self-evident that the seventh year is the year of release? Why then the apposition? To show that there is another release similar to this one, and that is a loan without a term which cannot be demanded before thirty days, as the master said that thirty days of a year count for an entire year."