ועתה כתבו לכם את השירה הזאת ולמדה את בני ישראל שימה בפיהם… (וילך לא:יט)

The passuk speaks about people who are weak in Emunah (faith in Hashem). They argue that one who is occupied learning Torah and performing mitzvahs has less parnassah (livelihood).  The passuk tells them: כתבו לכם את השירה הזאת – engrave the Torah in your hearts ולמדה את בני ישראל – and learn Torah day and night שימה בפיהם – and you will have plenty food and drink to place in your mouth, and will never lack parnassah – livelihood.