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.
ahmad:863Hāshim > Shuʿbah > ʿĀṣim b. Kulayb > Abū Burdah > ʿAlī

It is the middle finger, no doubt about it.  

أحمد:٨٦٣حَدَّثَنَا هَاشِمٌ حَدَّثَنَا شُعْبَةُ عَنْ عَاصِمِ بْنِ كُلَيْبٍ قَالَ سَمِعْتُ أَبَا بُرْدَةَ يُحَدِّثُ عَنْ عَلِيٍّ

أَنَّ رَسُولَ اللهِ ﷺ نَهَى أَنْ يُتَخَتَّمَ فِي ذِهِ أَوْ ذِهِ الْوُسْطَى وَالسَّبَّابَةِ وقَالَ جَابِرٌ يَعْنِي الْجُعَفِيَّ هِيَ الْوُسْطَى لَا شَكَّ فِيهَا