Unripe dates alone are forbidden, and unripe dates mixed with ripe dates are forbidden.
أخبرنا أحمد بن سليمان قال حدثنا يزيد قال أنبأنا حميد عن عكرمة
البسر وحده حرام ومع التمر حرام
No references are available.
[Sahih mawquf] It is found in Al-Kubra, hadith no. 5067, and it has corroborating narrations at hadith no. 3709, and in Ahmad, 1/310, 334, and elsewhere. Yazid is Ibn Harun.