Caution: Translations of Quran and Ḥadīth may lead to possible misapplications and misinterpretations. This site is intended for students of sacred knowledge that are proficient in comprehending classical Arabic and have a strong foundation in Islamic sciences. Also note that religious injunctions rely on several aspects beyond what one may glean through reading individual aḥādīth.
Qurʾān Bukhārī Muslim Nasāʾī Abū Dāwūd Tirmidhī Ibn Mājah Mālik Aḥmad Dārimī Ḥākim Ibn Ḥibbān Ṭabarānī Nasāʾī's Kubrá Bayhaqī al-Luʾluʾ wa-al-Marjān Ibn Rajab's Fifty Riyāḍ al-Ṣāliḥīn Aḥmad's Zuhd Suyūṭī
suyuti:4910a –

We saw the new moon of Ramadan as we were at Dhit-i-'Irq. We sent a man to Ibn Abbas (Allah be pleased with both of them) to ask him (whether the sighting of a small moon had something of the nature of defect in it). Upon this Ibn 'Abbas (Allah be pleased with both of them) said that the Messenger of Allah ﷺ had said: Verily Allah deferred its sight, but if (the new moon) is hidden from you, then, complete its number (thirty). (Using translation from Muslim 1088b)   

السيوطي:٤٩١٠a –

"إِنَّ الله ﷻ قَدْ أمَدَّهُ لرُؤْيته فَإنْ أُغْمى عَلَيكمْ فَأكْملُوا الْعِدَّةَ ".  

[ط] الطيالسي [حم] أحمد [م] مسلم عن ابن عباس

See similar narrations below:

Collected by Muslim, Aḥmad
muslim:1088b – [Chain 1] Abū Bakr b. Abū Shaybah > Ghundar > Shuʿbah [Chain 2] Ibn al-Muthanná And Ibn Bashhār > Muḥammad b. Jaʿfar > Shuʿbah > ʿAmr b. Murrah > Abū al-Bakhtarī > Ahlalnā Ramaḍān And Naḥn Bidhāt ʿIrq Faʾarsalnā a man > Ibn ʿAbbās Yasʾaluh > Ibn ʿAbbās

We saw the new moon of Ramadan as we were at Dhit-i-'Irq. We sent a man to Ibn Abbas (Allah be pleased with both of them) to ask him (whether the sighting of a small moon had something of the nature of defect in it). Upon this Ibn 'Abbas (Allah be pleased with both of them) said that the Messenger of Allah ﷺ had said: Verily Allah deferred its sight, but if (the new moon) is hidden from you, then, complete its number (thirty).  

مسلم:١٠٨٨b – حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ حَدَّثَنَا غُنْدَرٌ عَنْ شُعْبَةَ ح وَحَدَّثَنَا ابْنُ الْمُثَنَّى وَابْنُ بَشَّارٍ قَالاَ حَدَّثَنَا مُحَمَّدُ بْنُ جَعْفَرٍ أَخْبَرَنَا شُعْبَةُ عَنْ عَمْرِو بْنِ مُرَّةَ قَالَ سَمِعْتُ أَبَا الْبَخْتَرِيِّ قَالَ أَهْلَلْنَا رَمَضَانَ وَنَحْنُ بِذَاتِ عِرْقٍ فَأَرْسَلْنَا رَجُلاً إِلَى ابْنِ عَبَّاسٍ ؓ يَسْأَلُهُ فَقَالَ ابْنُ عَبَّاسٍ ؓ

قَالَ رَسُولُ اللَّهِ ﷺ إِنَّ اللَّهَ قَدْ أَمَدَّهُ لِرُؤْيَتِهِ فَإِنْ أُغْمِيَ عَلَيْكُمْ فَأَكْمِلُوا الْعِدَّةَ  

ahmad:3515 – Rawḥ > Shuʿbah > ʿAmr b. Murrah > Abū al-Bakhtarī > Tarāʾaynā Hilāl Shahr Ramaḍān Bidhāt ʿIrq Faʾarsalnā > Ibn ʿAbbās Nasʾaluh > In Nabī Allāh ﷺ

[Machine] The Prophet of God ﷺ said, "Indeed, Allah Almighty has extended the period for sighting the moon. So, if it is cloudy and you cannot see it, then complete the count (of days)."  

أحمد:٣٥١٥ – حَدَّثَنَا رَوْحٌ حَدَّثَنَا شُعْبَةُ حَدَّثَنَا عَمْرِو بْنِ مُرَّةَ عَنْ أَبِي الْبَخْتَرِيِّ قَالَ تَرَاءَيْنَا هِلَالَ شَهْرِ رَمَضَانَ بِذَاتِ عِرْقٍ فَأَرْسَلْنَا إِلَى ابْنِ عَبَّاسٍ نَسْأَلُهُ فَقَالَ

إِنَّ نَبِيَّ اللهِ ﷺ قَالَ إِنَّ اللهَ ﷻ قَدْ مَدَّهُ لِرُؤْيَتِهِ فَإِنْ أُغْمِيَ عَلَيْكُمْ فَأَكْمِلُوا الْعِدَّةَ