Why isn't there a source for a widow's time limit?
The Mishnah in Ketubot 57a.10 says:
נותנין לבתולה שנים עשר חודש משתבעה הבעל לפרנס את עצמה, וכשם שנותנין לאשה כך נותנין לאיש לפרנס את עצמו. ולאלמנה שלשים יום
One gives a virgin twelve months from the time the husband asked to marry her after having betrothed her, in order to prepare herself with clothes and jewelry for the marriage. And just as one gives a woman this amount of time, so too does one give a man an equivalent period of time to prepare himself, as he too needs time to prepare for the marriage. However, in the case of a widow, who already has items available from her previous marriage, she is given only thirty days to prepare.
The Gemara explains the reasoning behind the 12 months that a virgin gets on Ketubot 57b.2:
מנא הני מילי? אמר רב חסדא דאמר קרא (בראשית כד, נה) ויאמר אחיה ואמה תשב הנערה אתנו ימים או עשור
The Gemara asks: From where are these matters derived, that a virgin is given twelve months to prepare for her wedding? Rav Chisda said it is based on the fact that the verse states with regard to Rivka: “And her brother and mother said: Let the damsel abide with us for days, or ten” (Genesis 24:55).
However, the Gemara never explains the source for why a widow gets 30 days.
The gemara explains why she gets less time, but doesn't explain why 30. ↩︎