The Bible does not exactly say that, but –
• Jesus taught his disciples to always pray and not give up. Lk 18:1-8
• Paul wrote to “pray continuously” (without ceasing) 1 Thes 5:17
• Samuel said, “As for me, far be it from me that I sin against God by failing to pray for you.” 1 Sam 12:23
• Jeremiah reported what the Lord said, “Pray to Me . . . and I will be found by you.” Jer 29:12-14
• From Hebrews we learn, “He is a rewarder of those who diligently seek Him.” Heb 11:6
• James teaches that, “Anyone, then, who knows the good he ought to do and doesn’t do it, sins (sin of omission).” James 4:17 And, “Draw near to God, and He will draw near to you.” 4:8 And, “Pray for each other so that you may be healed.” 5:16
If failing to do something we are commanded to do is sin, then prayerlessness is surely a major sin for believers. On the other hand, if following the commands of the Lord is how we express our love for him then praying and interceding is an expression of love for God and love for those for whom we pray.