Midrasch zu Tehillim 79:8

2 Rabi Abahu says it says (Exodus 23,4): 'Three pilgrimages you will celebrate for me in the year,' The Holy One established three pilgrimages and in the merit of the fathers that don't come before The Holy One empty-handed, and even in these generations even though they are troubled (narrow, stressed), the Torah has not moved from them as it says (Isiah 59,21): 'Should not be removed from your lips or your offspring lips, or your offspring's offsprings lips...'.