Bolt holding saddle to seatpost broke
You need to use high tensile bolts for high tensile positions like this.

A class 8.8 bolt (metric) or grade 8 (US) or higher is appropriate.   Cheese-grade steel from the local discount hardware store is not up to the task.

 US Grade 8 head marking.  These have a min yield strength of 130,000 PSI. (900 MPa)

 Metric Class 8.8 head marking, with a min yield strength of 640 MPa (93,000 PSI)



Your second option is to replace the seatpost with one that has two bolts for fixing,  This will decrease the flex possible on one bolt.

  One style of two-bolt saddle clamp, but there are multiple other designs.   They're all integrated with seatpost so its got to be exactly the same diameter as the seatpost you're replacing.



Underlying - your monthly maintenance check on your bike should have tested the saddle wobbliness and noticed any increase in play.  At that time you'd retighten the bolt to torque.  If it was over-torqued a low-grade bolt could have stretched and deformed, leading to eventual failure.



Finally - be a responsible road user and put the phone away.  Ride with your hands on the bars.  If not for your own safety then for others on the road with you.