Sayyidah Umm al-Darda' (may Allah be pleased with her) reports this as a marfu' hadith (the Prophet (peace and blessings be upon him) said): "Whoever stands guard at any of the Muslims' coasts for three days, that suffices him for a year of standing guard (i.e., he receives the reward of a year of guarding)."
Ahmad and al-Tabarani recorded it through the narration of Isma'il bin Ayyash from the people of Madinah, and the rest of its narrators are thiqat (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الجهاد / 9496
Hadith Gradingمحدثین:إسنادہ ضعیف
Hadith Takhrij«اخرجه الطبراني فى المعجم الكبير (254/24) اخرجه احمد فى المسند (362/6) ، أخرجه مسند أحمد برقم: 26500»