It is common knowledge that Moshe Rabeinu was Niftar on 7 Adar. Was it a plain year with 1 Adar or a leap year with 2 Adars? If it was a leap year on what Adar was he Niftar?
The Gemara (Kiddushin 38a) deduces the date of Moshe's passing by calculating backwards from the 10th of Nissan, when the Jewish People crossed the Jordan River: there were 30 days of mourning for Moshe, and three days of preparing for the crossing. Thirty-three days before the 10th of Nissan gets us to the 7th of Adar.
Given that, then, I would think that obviously it must mean plain Adar or Adar Sheni; if it was the 7th of Adar Rishon, then that would be 63 days before the 10th of Nissan.
(Come to think of it: since he began speaking Chumash Devarim on the 1st of Shevat and continued doing so for 36 days, until his passing (see Ohr Hachayim to Deut. 1:1), that also would mean that it can't have been a leap year, otherwise the number of days from 1 Shevat to 10 Nissan would be more than 69.)
Edit: thanks to Menachem for referring me to Midrash Sechel Tov on Ex. 16:35, which cites several views on the subject (taken from Mechilta on this verse). R' Yehoshua places Moshe's passing on 7 Adar (he doesn't say whether plain Adar or Adar II); R' Elazar Hamoda'i says it was 7 Adar I; and R' Eliezer says it was 6 Shevat in a regular year. Sechel Tov accepts the opinion of R' Elazar Hamoda'i, and explains that indeed they cried for Moshe's passing for thirty days and then mourned him for another thirty (as per Sifri to Deut. 34:8).
I don't know how much this source is reliable, but it is explained there that Moshe Rabeinu was born on a simple year, and was niftar on 7 Adar beis of a leap year.
Rabbi Acha ben Chanina says that on 6 Sivan was the day Moshe was put in a basket into the water. The Malachim said to Hashem "The one that is going to receive the Torah on Har Sinai on this day should get smitten by the water on this day?" 6 Sivan was 3 months after Moshe was born. Based on the above you have to say that he was born in either a plain Adar or on Adar Beis. Thus it would follow that he passed away either on a plain Adar or on Adar Sheni - as Hakodosh Boruch Hu Memalei Shenoison Shel Tzadikim.
However the Sefer HaTodaa mentions in the name of the Yalkot Yehoshua that Moshe passed away on 7 Adar Rishon.