Book of Jubilees 28:20
And when Leah saw that she had become sterile and did not bear, she envied Rachel, and she also gave her handmaid Zilpah to Jacob to wife, and she conceived, and bare a son, and Leah called his name Gad, on the twelfth of the eighth month, in the third year of the fourth week. [2131 A.M.]
Background and history of Book of Jubilees, and issues with this book.
R. H. Charles, The Book of Jubilees, in The Apocrypha and Pseudepigrapha of the Old Testament, vol. 2 (Oxford: Clarendon Press, 1913). Public domain in the United States (published 1913). Charles died in 1931, so the book is in the public domain in the United Kingdom. The verse text is the Joshua Williams transcription at pseudepigrapha.com. Jubilees 1:8 and 1:9 are divided at the break in the 1902 Clarendon scan (archive.org bookofjubileesor00char), where verse 9 begins “For they will forget.” That scan also numbers the angel-of-the-presence sentence as 1:29. Editorial brackets in the translation are Charles’s.