ET first learned that the co-stars turned couple were dating back in January.
"Bill and Anna are in love and very happy in their relationship, and enjoying their time together," the source shared.
While the lovebirds appear to be in relationship bliss, the source said that Hader and Kendrick are not getting ready to head down the aisle just yet.
"They're in no rush to get engaged and are happy with how things are going," the source added.
ET first learned the pair were an item back in January, when a source told ET that the 36-year-old actress and 43-year-old Saturday Night Live alum had been "dating for a while" now.
"Anna and Bill have been dating for a while. They really like each other and share the same sense of humor," the source says. "They're both very happy in the relationship and it shows. They have a natural connection."
Hader and Kendrick starred in the 2019 Disney+ Christmas movie, Noelle, together, but it's unclear when they officially began dating.
Hader was most recently linked to The Hart of Dixie star, Rachel Bilson, but the pair called off their relationship in July 2020, just six months after making their red carpet debut at the 2020 Golden Globes.
The two actors co-starred in The To Do List in 2013, which was written and directed by Hader's then-wife, Maggie Carey. Hader and Carey split in 2017, and share three daughters together: Hannah, 12, Harper, 9, and Hayley, 7.
Kendrick, on the other hand, had been in a relationship with British cinematographer Ben Richardson since February 2014, which she kept private. She and Richardson were last photographed together in 2019.